Decreased cellularity of the thymus is the most frequently encountered histologic finding associated with compound-induced effects on the thymus. Because decreased cellularity is often associated with the histologic presence of dead lymphocytes it is important to attempt to distinguish between lymphocyte apoptosis versus necrosis. The presence, severity grade and location of cell death, when present, should be determined. The determination of the type of cell death is important because it may provide insight into the pathogenesis of the lesion. The STP Committee on the Nomenclature of Cell Death recommends the use of the term “necrosis” to describe findings comprising dead cells in histological sections, regardless of the pathway by which the cells died (Levin et al., 1999
). They also recommend the use of the modifiers “apoptotic” and “oncotic” to specify the predominant morphological cell death pathway.
Oncotic necrosis is the cellular process that can be seen in areas of thymus infarction or as a direct treatment-related effect and may or may not be accompanied by an inflammatory response rich in neutrophils. With oncotic necrosis, there is cell swelling and rupture of the cell membrane and subsequent release of cytoplasmic contents into the surrounding interstitium which incites the inflammatory response. Apoptotic necrosis on the other hand, is characterized by cell shrinkage, nuclear fragmentation, extrusion of membrane-bound cytoplasm and nuclear debris in the form of small dense apoptotic bodies. This process is typically accompanied by tingible body macrophages (defined as macrophages containing stainable cellular debris), which give the tissue a “starry sky” appearance (). It is therefore recommended that, whenever possible, a diagnosis of lymphocyte oncotic necrosis be reserved for those cases where treatment results in the classical form of necrosis rather than apoptosis.
Figure 1 Figures 1a and 1b are low and high magnifications of normal thymus cortex from a control 30-day-old male Sprague–Dawley rat. These can be compared with the low and high magnifications of thymus cortex in Figures 1c and 1d from an age- and sex-matched (more ...)
As noted before, decreased numbers of lymphocytes in the thymus, leading to decreased cell density, decreased cellularity, or decreased compartment size, may be the result of direct thymic lymphocyte toxicity or may result from endogenous glucocorticoid release or age-associated involution. Certain chemicals that lead to direct thymus lymphocyte toxicity may result in increased numbers of apoptotic lymphocytes and tingible body macrophages (Figures and ) or may result in oncotic necrosis (). While not an absolute truth, apoptotic necrosis is most likely to occur as a secondary response to stress, while that of oncotic necrosis may be considered to be more representative of direct lymphocyte (thymocyte) toxicity. However, apoptotic necrosis and oncotic necrosis are not mutually exclusive processes and thus may occur simultaneously since both represent morphologic expressions of a shared biochemical network (Zeiss, 2003
Figure 2 Figure 2a is a transmission electron photomicrograph of normal lymphocytes from the thymus cortex of a control 30-day-old male Sprague–Dawley rat. Figure 2b shows apoptotic lymphocytes from the same region in a rat treated 3 hours previously with (more ...)
Figure 3 Figure 3a is an image of cortical necrosis in a 30-day-old male Sprague–Dawley rat treated 48 hours previously with 1 mg/kg dexamethasone. The large arrow indicates an enlarged medullary region that is being repopulated with small, mature lymphocytes. (more ...)
Endogenous glucocorticoid release in response to stress and debilitation can occur within a group of animals and this can result in increased numbers of thymus cortical apoptotic lymphocytes. However, lymphocytes in the cortex normally undergo numerous cell divisions before entering the medulla and apoptosis is a normal but usually minimal finding in this population of rapidly dividing cells. Therefore, an increase in the number of apoptotic cells should be noted only after comparison with controls. Although various methodologies are available for evaluating apoptosis (stained resin sections, DNA laddering, TUNEL, annexin V, caspase-3 activity as-says, mitochondrial assays, vital dyes and lysotracker red), each assay has its advantages and disadvantages that can render it appropriate and useful for one application but inappropriate or difficult to use in another (Watanabe et al., 2002
). Transmission electron microscopy () is considered the “gold standard” for the evaluation of apoptosis.
Spontaneous aging and end-stage or chronic experimentally induced non-neoplastic thymic lesions are often morphologically similar with reduction in thymic weight and histological depletion of cortical lymphocytes. Therefore comparison with age-matched controls is crucial. The mechanisms responsible for age-related thymic involution are not known however sex hormones are involved. Orchidectomy in rats will cause involutional effects on the accessory sex organs with trophic effects on the thymus due to removal of the sex hormones. Treatment of old male rats with a stable analogue of luteinizing hormone-releasing hormone (LHRH) and the subsequent decrease in testosterone has been shown to result in regeneration of the thymus (Greenstein et al., 1987
). Estrogen also has significant immunomodulatory properties, including induction of thymic involution (Yao and Hou, 2004